1. Field of Invention
The present invention relates to the application of a solution processed polycyclic aromatic compound precursors as an organic semiconducting material in thin film transistors. More particularly, present invention relates to a pentacene precursor that is a hetro Diels-Alder adduct of pentacene with heterodienophiles having at least one heteroatom selected from N, O, or sulfur. These adducts are highly soluble in common organic solvents and can be easily converted back to pentacene in solid state by heating at moderate temperatures.
2. Background of the Invention
Thin-film transistors (TFT) and other electronic devices using organic semiconductors are emerging as alternatives to established methods using amorphous silicon (α-Si:H) as the semiconductor.
Amorphous silicon (α-Si:H) is typically used in thin film field-effect transistors in many applications including liquid crystal displays. In the past decade significant efforts have been directed towards development of organic semiconductors to replace amophous silicon in low-end applications. The organic materials provide the ability to produce these devices bypassing the expensive steps associated with silicon processing.
Other advantages of organic semiconductors include low temperature processing combined with ease of deposition of thin films of these materials over large areas. These processing characteristics enable the fabrication of organic thin film transistors (OTFT) on plastic substrates, which offer light weight, mechanical flexibility and ruggedness.
A variety of organic materials have been designed, synthesized and characterized as p-type semiconductors (majority carriers are holes). Organic thin film transistor (TFT) devices have been made using such materials. Among them, thiophene oligomers have been proposed as semiconducting materials in Garnier et al., “Structural basis for high carrier mobility in conjugated oligomers” Synth. Met., Vol., 45, p. 163 (1991). Benzodithiophene dimers are proposed as organic semiconductor materials in Liquindanum, J. et al., “Benzodithiophene rings as semiconducting building blocks” Adv. Mat., Vol. 9, p. 36 (1997).
Pentacene, which is a member of polyacene, is one of the most widely studied organic semiconductor and is proposed as semiconducting material for TFT devices in Dimitrakopoulos et al., “Molecular Beam Deposited Thin film of Pentacene for Organic Field-effect Transistor Applications,” J. Appl. Phys., 1996, 80, pages 2501-2508, Jackson et al., “Pentacene Organic Thin-film Transistors for Circuit and Display Applications”, IEEE Trans. Electron Devices 1999, 46, pages 1259-1263, Dimitrakopoulos et al., “Fabrication of Thin Film Field Effect Transistor Comprsing an Organic Semiconductor and Chemical Solution Deposited Metal Oxide Gate Dielectric,” U.S. Pat. No. 5,946,551 and Dimitrakopoulos et al., “Low Temperature Thin Film Transistor Fabrication,” U.S. Pat. No. 6,207,472 B1.
Pentacene has shown the highest field effect mobility (greater than 1 cm2 V−1 sec−1) and high on/off ratio (greater than 107) among organic semiconductors. The fact that the mobility is comparable, to amorphous silicon (α-Si:H) indicates that pentacene may be applied in large-area microelectronic applications such as TFT backplanes for active matrix displays and low end and low cost integrated circuits. However, for an organic semiconductor technology to be a viable low cost alternative to amorphous silicon, it must be soluble in organic solvents to be suitable for application on large areas by inexpensive methods, such as, stamping, screen printing and spin-coating.
Pentacene is virtually insoluble in organic solvents. Presently, pentacene thin films are deposited by costly high vacuum techniques for application in TFT and other electronic devices. Although there has been one example in the literature to form pentacene films through a soluble precursor of pentacene (see Muellen et al., “A soluble pentacene precursor: Synthesis, solid state conversion to pentacene and application in a field-effect transistor,” Adv. Mat., Vol. 11, 1999, pages 480-483), the lengthy synthetic approach to the pentacene precursor with very low overall yield makes that approach impractical and commercially unattractive.
Therefore, it is an object of the present invention to prepare thin films of a polycyclic aromatic compound, such as, pentacene, by solid state conversion of thin films of an organic solvent soluble precursor thereof to the polycyclic aromatic compound.
It is another object of the present invention to prepare thin films of an organic solvent soluble precursor of a polycyclic aromatic compound by applying a solution of the precursor in an organic solvent onto a substrate and thereafter removing the solvent.
It is still another object of the present invention to use soluble precursors of a polycyclic aromatic compound, which is a Diels-Alder adduct of the polycyclic aromatic compound with a hetero dienophile, such as, dialkyl azodicarboxylate, acyl nitroso compound, N-sulfinyl amide, thioxocarboxylate and malonate.
It is yet another object of the present invention to use a thin film of a polycyclic aromatic compound, such as, pentacene, in a thin film transistor in which the polycyclic aromatic compound film acts as a p-type semiconductor channel.
It is further another object of the present invention to provide films of a polycyclic aromatic compound, such as, pentacene, for the fabrication of TFT devices that are prepared by a low to moderate temperature conversion of a precursor thereof to the polycyclic aromatic compound at about 80° C. to about 250° C. in a retro-Diels-Alder reaction.
None of the above references describes precursors of polycyclic aromatic compounds that are: (1) Diels-Alder adducts of a polycyclic aromatic compound, such as, pentacene, with a dienophile; (2) highly soluble in common organic solvents; and (3) used to prepare a thin film transistor in which the polycyclic aromatic compound film acts as a p-type semiconductor channel.
The present invention provides highly soluble precursors of polycyclic aromatic compounds, such as, pentacene, which are synthesized in one step via the Diels-Alder reaction of polycyclic aromatic compound with a variety of dienophiles having at least one heteroatom in the dienophile moiety and are used in the fabrication of TFT devices.